Mobile communication systems operate on a cellular basis in which transmission and receiver base stations coupled to a network provide transmission and reception to user equipment (UE) such as mobile telephones in each cell in a network. There is usually a single base station in each cell. The base stations are usually referred to as macro base stations and the cells as macrocells.
Smaller cell sites called picocells have been proposed. These cover a smaller area such as a complex of buildings. Still smaller cell sites called femtocells are also proposed. This is a term used by mobile operators to refer to cell sites of the type which attempt to solve the often expensive problem of providing complete in-building coverage where macrocells are often unable to provide adequate coverage due to attenuation and scattering of the radio signal from the base station.
A femtocell is often referred to as an access point base station or home gateway. It is a small plug and play device which communicates with user equipment such as mobile handsets using standard 2G or 3G transmission reception. It is connected to the cellular network via a broadband service using either Xdsl or WiMax technology. Xdsl is a family of technologies in which a digital subscriber loop allows broadband communications over conventional copper telephone lines, and WiMax, is a wireless Technology defined by the IEEE 802.16 standard providing improved wireless broadband compared to conventional Wi-Fi® systems. WiMax™ is a pending trade mark application of the WiMax Forum, California, USA and Wi-Fi® is a registered trade mark of the Wi-Fi® Alliance, California, USA.
A femtocell can optionally incorporate the functionality of a broadband router so that a user has a completely integrated device. It is of course essential that the femtocell integrates seamlessly with the core network of the mobile communication system so that it can be remotely managed and updated.
Femtocells are typically implemented using second generation (2G) and third generation (3G) wireless cellular networks. In Europe, one of the common 3G technologies, Universal Mobile Telecommunications System (UMTS), uses Wideband Code Division Multiple Access (W-CDMA) as the Radio Access Technology (RAT) to provide wireless communication. However, other RATs such as Code Division Multiple Access 2000 (CDMA 2000), Time Division Multiple Access (TD-CDMA), Universal Wireless Consortium (UWC), or Digital European Cordless Telecommunication (DECT) radio technologies may be used to implement 3-G networks.
In W-CDMA implemented 3G networks, there are two modes of data transmission; frequency division and time division. In the frequency division mode, communications between a mobile communication device and base station are provided in the frequency division mode by assigning one particular frequency for the uplink to the base station, and another frequency for the downlink from the base station to the mobile communication device. In this way, a mobile communication device is allocated part of the frequency spectrum for the entire duration of the communication. In the time division mode of operation, each user is allowed to transmit only for a length of time defined by an allocated slot, but is allowed to use the entire bandwidth of the channel for that transmission.
Femtocells generally provide wireless networks which typically cover small areas, such as residential environments, and as such they tend to offer limited bandwidth and therefore capacity in terms of the number of mobile communication devices that can simultaneously use the femtocell for wireless communication. The capacity of a femtocell to simultaneously handle voice calls may be as low as 4 voice calls at any one time. Consequently, as the number of mobile communication devices accessing or attempting to access the femtocell increases, there is a need to redirect devices from the femtocell to other neighbouring networks.
Known Third generation Partnership Project (3GPP) specifications provide a redirection feature mechanism for W-CDMA/UMTS.
The redirection feature allows a femtocell to transfer mobile communication devices which are using the femtocell for wireless communications to another cell operating under another RAT or to another frequency in the same 3G RAT.